Techniques of forming a visible image from video information by way of an electrostatic latent image are currently in wide use. Electrophotography, a typical example of these techniques, consists essentially of a charging and exposure step wherein an electrostatic latent image formed on a support made of a photoconductive photoreceptor is developed with a developer made of colored electroscopic particles, a transfer step wherein the resulting toner image is transferred and fixed to a receiving sheet to form a visible image, and a post-imaging step wherein the support having the transferred image is cleaned in preparation for another cycle of image formation.
Several requirements must be met to form visible images of consistent quality over an extended period, and the developer used in developing an electrostatic latent image must also satisfy certain conditions. Conventionally, the developer is made of a powder of electroscopic colored particles having a colorant dispersed in a binder resin (this powder is hereunder referred to as a toner powder). To form a visible image of good quality, the developer must have high fluidity, and for this purpose, the conventional toner powder contains fine particles of silicon dioxide. But raw fine particles of silicon dioxide are hydrophilic and the developer containing them experiences block ing (individual particles agglomerate) due to the moisture of air with the result that its fluidity is decreased. To avoid this problem, proposals have been made for using fine particles of silicon dioxide after they have been rendered hydrophobic see Japanese patent applications (OPI) Nos. 5782/71, 47345/73 and 47346/73 (the symbol OPI as used herein means an unexamined published Japanese patent application) . According to these proposals, silicon dioxide particles and an organosilicon compound such as dimethyl dichlorosilane are fed concurrently with water vapor by an inert gas such as nitrogen into a reactor filled with a heated fluidized bed, thereby reacting the silanol groups on the silicon dioxide particles with the organosilicon compound to render the silicon dioxide particles hydrophobic. But the hydrophobicity obtained is not satisfactory; in a hot and humid atmosphere, the toner particles have a tendency to agglomerate and their fluidity is decreased. In addition, wet toner particles easily let leak static charges and the reduced charge buildup leads to a visible image of impaired quality.
The conventional particles of hydrophobic silicon dioxide are considerably hard and have a great chance to damage the support for an electrostatic latent image. This is conspicuous when the support is made of a selenium-tellurium system or an organic photoconductive material and is relatively soft. A support made of a selenium-arsenic system which is relatively hard but is very vulnerable to mechanical shock is also liable to be damaged. The damage to the support is particularly great when the support is subjected to high-speed contact cleaning wherein it is cleaned with a conforming wiper blade or a fur brush. If the damage to the support goes to an extreme, the blade or fur brush is also damaged to make further cleaning difficult.
As well known, the deposition of toner particles on the support and subsequent damage to its surface can be effectively prevented by adding a lubricant such as a fatty acid metal salt (e.g. zinc stearate) to the toner powder. Since the fluidity of the toner containing a lubricant is appreciably reduced, the addition of fine silicon dioxide particles is generally necessary. But then a toner containing both a waxy lubricant and fine silicon dioxide particles has a particularly great tendency to damage the support. This defect is especially conspicuous when the support is made of a relatively soft material such as a selenium-tellurium system or an organic photoconductive material or a hard but brittle material such as a selenium-arsenic system, or when the support is cleaned with a conforming a doctor blade or a fur brush.
As described above, the conventional toner containing fine particles of hydrophobic silicon dioxide becomes less fluid with time and experiences blocking. In addition, the toner presents damage to the support for an electrostatic latent image. Therefore, it is difficult to produce visible images of consistent quality with this toner over an extended period.